Among the most crucial tasks of vision science is to determine the nature of the basic units over which visual processes operate. Much recent evidence has accumulated that processes of visual attention can in some cases operate over visual objects. The goal of the proposed research is to determine the nature of these visual objects, moving beyond the simple stimuli (static 2D dots, bars, and boxes) typically used in previous research. What is an 'object', in the context of 'object based visual attention'? Several experimental techniques (including change detection, multi-element tracking, spatial cueing, and eye- movement monitoring) will be employed with a broader and more naturalistic set of stimuli (focusing on perceptual groups, rendered 3D volumes, and dynamic 'object schemas'), in the context of various dynamic behaviors (including various degrees of spatiotemporally-continuous motion). This research will uncover the nature of the basic units of visual attentional processing in normal subjects, and will shed light on the nature of various object-based disorders, such as unilateral neglect and Balint syndrome.